The reservoir type dispenser cathode comprises an electron emissive material made by press-molding tungsten and barium calcium aluminate, a porous metal base body positioned on the upper portion of the electron emissive material and provided with the diffusing cavity for diffuse Ba, a container storing the electron emissive material, and a sleeve supporting and fixing said container and enclosing the heater.
Some additives are added to the porous metal base body and the electron emissive material based on the above mentioned basic structure or material in order to lower the operating temperature of the cathode or enhance the current density. For example, as described in U.S. Pat. No. 4,823,044, issued to Ceradyne, Inc., suitable amount of Ir, Os, Ru, Re, etc., permeates into the porous metal base body. This cavity reservoir type dispenser cathode is inexpensive in manfacturing cost and has the current density of over 10 A/cm.sup.2.
But the aforesaid cavity reservoir type dispenser cathode is disadvantageous in that the time required for activation aging i.e., the time required for forming monatomic layer on the inner wall and the surface of the cavity of the porous metal base body is as long as approximately 10 to 30 hours, thereby decreasing the productivity of the product. The reason why the time required for the activation aging is lengthened is that diffuse Ba from the electron emissive material is diffused gradually through the cavity of the porous metal base body positioned on the electron emissive material and lastly it reaches the surface of the porous metal base body. In more detail, when diffuse Ba generated by thermal energy from the heater passes through the cavity and the monatomic layer is formed gradually on the surface of the porous metal body, the monatomic layer is not formed on the surface of the porous metal base body until Ba layer is sufficiently formed on the inner wall of the cavity (i.e. until the concentration thereof reaches the state of the saturation.).
To overcome these problems, there is a method to increase the produced amount of Ba. However, this method should increase the heat amount generated from the heater and therefore may shorten the lifetime of the heater and vaporize excessive amount of Ba. Thus, the lifetime of the cathode itself i.e. the time which can maintain the thermal electron emission for a long period may be short. Further, if vaporized Ba which does not contribute to form the monatomic layer is attached to a part of the periphery of the cathode, the lowering of the performance and the deterioration of the product itself are resulted.